This invention generally relates to structures and methods for the suppression of unwanted radio frequency and microwave signals on circuit boards, and more particularly to the use of a ferrite layer to couple with unwanted radio frequency and microwave signals in circuit boards.
Circuit boards are used in a wide variety of electronic equipment such as radio and television sets, remote controls, calculators, telephones, cellular phones, personal computers, personal digital assistants, automobiles, manufacturing equipment, and many other applications where circuitry for electronic applications are needed. Printed circuit boards and flexible circuits are used in many applications because they are lightweight, inexpensive to produce, small, and readily produced in a uniform fashion in large numbers.
Printed circuit boards have one or more layers of wiring printed on respective layers of insulating material. Wiring can be present on two sides of a single layer of insulation with throughholes, eyelets or solid conducting connector. The wiring is often copper, but can also be any other conductive metal, and can be imparted by screening, vacuum deposition, plating and other techniques well known to those of ordinary skill in the art. The insulation can take a variety of forms such as resin and fiber laminate (prepreg), epoxy, ceramic, and plastic films such as polyimide films. The insulation that the circuit is printed on is often coated with another insulating layer to protect the circuitry. Circuit boards can have a number of other components either printed thereon or otherwise attached. Such components can be almost any type of electronic components and can include heat dissipating elements, integrated circuit chips, resistors, capacitors, inductors, transformers, diodes, transistors, oscillators, operational amplifiers, integrated amplifiers, power supplies, and voltage regulators to name but a few.
Some components on a circuit board can generate radio frequency (xe2x80x9cRFxe2x80x9d), with frequencies of less than about 30 MHz, or microwave signals, with frequencies of about 30 MHz up at least 300 GHz presently and up to at least 3,000 GHz in the future. These components on or near the circuit board will impart RF or microwave signals or both to the circuit board. Such signals can undesirably affect the operation of other components, devices, or systems on the circuit board or other nearby circuit boards, and as is often referred to as electromagnetic interference (EMI). Electromagnetic interference problems can be particularly insidious as parts that appear innocuous act as resonant circuits and have enormous effective cross-sections for pickup of external electromagnetic signals. Horowitz and Hill, The Art of Electronics, p. 457 (Cambridge University Press 2d. ed. 1980). The increasing use of microwave signals in communications makes microwave interference more common with time.
Accordingly, many electronic devices, such as transmitters, receivers, amplifiers and the like require some method of stripping unwanted RF and microwave signals from power and signal processing circuitry present on circuit boards. In the past, one approach for accomplishing this on circuit boards uses of lumped element ferrite beads and/or toroids made from lumps of ferrite. Such lumped ferrite elements can be costly to purchase and assemble on the finished circuit board. Additionally, lumped elements may not be entirely effective in suppressing unwanted EMI. First, the suppression is only effective in the immediate vicinity of the lumped ferrite element. Second, effective lumped elements can be large in comparison to space available on a circuit board or the distance available for installation.
Further, correctly suppressing unwanted EMI with lumped ferrite elements can be difficult. Often during the development of a circuit board product, small changes can be made. Such small changes to the design of a circuit board can have unpredictably large changes in the effectiveness of the suppression offered by the lumped ferrite element. These unexpectedly large changes in the effectiveness of the suppression can result in long and expensive development cycles for products.
Another method for protecting sensitive components uses a metal, such as copper or silver, to shield portions of a printed circuit board from other portions of a printed circuit board. Such methods often reflect radio frequency or microwave radiation rather than absorb it. Structures implemented in accordance with such methods can act as waveguides for radio or microwave radiation and exacerbate problems in the radio frequency or microwave spectra rather than solve problems related to electromagnetic interference. To the extent such methods succeed, the methods require the absorbing metal to be grounded to a sink to draw the electromagnetic radiation awayxe2x80x94xe2x80x94the metals themselves do not dissipate unwanted signals.
Accordingly, there exists a need for technology that permits the effective suppression of unwanted RF and microwave signals on circuit boards. Desirably, such technology will have inexpensive components. It is also advantageous that such technology not present difficulties in locating components on the printed circuit board. Even more desirably, such a technology will be easily implemented so that small alterations to the design of a printed circuit board do not require time consuming and expensive troubleshooting and/or redesign of the printed circuit board. Such methods will not provide structures that act as wave guides for the unwanted signals, and will not require further components or grounding to protect sensitive components from electromagnetic interference.